Hypertension is a treatable major cardiovascular (CV) risk factor, which afflicts 30% of American adults. Despite many classes of antihypertensive drugs, control rates are poor, leading to CV events, renal failure, and stroke. The most common form of secondary hypertension is primary aldosteronism (PA), with a prevalence of 8-10% amongst all hypertensive patients and 14-30% among patients with resistant hypertension. Despite facile identification using aldosterone/renin ratio (ARR) and readily available treatment with directed therapies, screening rates for PA are extremely low (<0.1% of all hypertensives). The reasons for this missed opportunity include the long, complex, and difficult evaluation of PA. Accurate subtyping of PA into aldosterone-producing adenoma (APA, unilateral 30% of PA) or idiopathic hyperaldosteronism (IHA, bilateral, ~65% of PA) is essential to guide treatment. Surgery is recommended for most APAs and mineralocorticoid receptor antagonist (MRA) for IHA. Adrenal vein sampling (AVS) is the only test that reliably distinguishes APA from IHA, but few centers perform this technically challenging procedure well. A simple blood test to identify surgically curable cases of PA would be a major advance in the management of hypertension, particularly patients with resistant hypertension. Our basic research studies have shown that nearly all APAs express some steroidogenic enzymes not normally found in aldosterone-producing cells. Based on that finding, we then showed that unusual hybrid steroids and precursors, such as 18-hydroxycortisol (18OHF) are more abundant in serum from patients with APA than those with IHA. However, the normal adrenal makes small amounts of some of these steroids, leading to overlap and poor discrimination. By combining dynamic testing (saline or dexamethasone suppression) and steroid profiling using mass spectrometry, we will identify steroids and ratios, which reliably distinguish APA from IHA patients. This proposal will develop a clinically useful diagnostic test that will define patients who DO NOT have APAs and eliminate their need for imaging and AVS. This work will encourage screening for PA and reduce the cost and risk of the evaluation. The findings from the proposed pilot studies will be used to develop multi-center trials of the testing algorithm, which will enhance the adoption and dissemination of this approach to improve patient care and outcomes.